Thanks. Just looked at the template for index.php and changed it to get what I want:
Where it displays the story content, put in an if check. On the front page the query string variable p isn't defined. So you can use this to stop it displaying the content for each story.
When you click on the link of the story title, p is defined.
I've modified index.php to only display the title and excerpt:
<?php the_content(); ?>
<?php if ($_GET[p] != "") the_content(); else the_excerpt(); ?>
if the excerpt is empty, it displays the content for some reason. Probably something to do with the_excerpt()